1. Field of the Invention
The present invention relates to shift register circuit only composed of same conductivity type field-effect transistors used in a scanning line drive circuit of an image display device, for example, and more particularly to a bidirectional shift register capable of inverting a shift direction of a signal.
2. Description of the Related Art
According to an image display device such as a liquid crystal display device (referred to as “display device” hereinafter), a gate line (scanning line) is provided for each pixel row (pixel line) of a display panel in which a plurality of pixels are arranged in the form of a matrix, and a display image is updated by sequentially selecting and driving the gate line based on one horizontal period of a display signal. As a gate line drive circuit (scanning line drive circuit) to sequentially select and drive the pixel line, that is, the gate line, a shift register performing a shift operation with respect to each frame period of the display signal may be used.
It is preferable that the shift register used in the gate line drive circuit be only composed of same conductivity type field-effect transistors to reduce a number of steps in a production process of the display device. Thus, various kinds of shift registers only composed of an N type or P type field-effect transistors and display devices mounting it are proposed. As the field-effect transistor, a MOS (Metal Oxide Semiconductor) transistor, a thin film transistor (TFT), and the like may be used.
The gate line drive circuit is composed of a plurality of stages of shift registers (multi-stage shift register). More specifically, the gate line drive circuit is configured by cascade-connecting a plurality of shift register circuits provided for each pixel line, that is, each gate line. In this specification, each shift register circuit constituting each stage of the multi-stage shift registers is referred to as “unit shift register”.
The matrix type liquid crystal display device in which the liquid crystal pixels are arranged in the form of a matrix is required to change a display pattern such as to invert the display image vertically or laterally, or change the display order at the time of display.
For example, display reversing is required when the liquid crystal display device is applied to projection equipment for OHP (Overhead Projector) on a translucent screen. When the translucent screen is used, since a video is projected from a back side of the screen as viewed from audience, the video on the screen is reversed as compared with the case where the video is projected from the front side of the screen. In addition, the display order change is required to stage-manage a display such as a bar chart and a histogram so that the display image appears gradually from an upper side to a lower side, or lower side to an upper side.
Switching of a shift direction (scanning direction) of the signal in the gate line drive circuit is one method for changing the display pattern of the display device. Thus, a shift register capable of switching the shift direction of the signal is proposed (for example, National Publication of Translation No. 11-502355). The shift register capable of switching the shift direction of the signal is referred to as “bidirectional shift register” and each stage is referred to as “bidirectional unit shift register”, occasionally.
A bidirectional unit shift register composed of N channel type field-effect transistors only is disclosed in FIG. 7 of National Publication of Translation No. 11-502355. The unit shift register has an output terminal connected to a first transistor (MN2) for supplying a clock signal (φ1) to the output terminal and to second and third transistors (MN4 and MN7) for discharging the output terminal. The second transistor (MN4) is driven by an output signal of a subsequent-stage (n+1) and the third transistor (MN7) is driven by an output signal of previous-stage (n−1).
The first transistor (MN2) is driven by fourth and fifth transistors (MN1 and MN3) connected to its gate node (G) (referred to as “node G”). The fourth transistor (MN1) is driven by the output signal of the previous-stage (n−1), and supplies a predetermined first voltage signal (Vb) to the node G. The fifth transistor (MN3) is driven by the output signal of the subsequent-stage (n+1), and supplies a predetermined second voltage signal (Vh) to the node G.
The first and second voltage signals (Vb and Vh) are complementary signals such that when one voltage level (referred to as simply “level” hereinafter) of one is at H (High) level, the other level is at L (Low) level. The shift direction of the signal in the bidirectional unit shift register is determined by the levels.
For example, in the case where the first voltage signal (Vb) is at H level and the second voltage signal (Vh) is at L level, when the output signal of the previous-stage (n−1) becomes H level and the fourth transistor (MN1) is turned on, the node G becomes H level and the first transistor (MN2) is turned on. Thus, the output signal is outputted at the timing when the clock signal (φ1) becomes H level next. That is, when the first voltage signal is at H level and the second voltage signal is at L level, the corresponding unit shift register operates so as to output the signal after the its previous-stage (n−1) (this operation is referred to as “forward shift”).
Meanwhile, in the case where the first voltage signal (Vb) is at L level and the second voltage signal (Vh) is at H level, when the output signal of the subsequent-stage (n+1) becomes H level and the fifth transistor (MN3) is turned on, the node G becomes H level and the first transistor (MN2) is turned on. Thus, the output signal is outputted at the timing when the clock signal (φ1) becomes H level next. That is, when the first voltage signal is at L level and the second voltage signal is at H level, the corresponding unit shift register operates so as to output the signal after its subsequent-stage (n+1) (this operation is referred to as “backward shift”).
Thus, according to the conventional bidirectional unit shift register (in FIG. 7 of National Publication of Translation No. 11-502355), the shift direction of the signal is switched by switching the levels of the first and second voltage signals (Vb and Vh) supplied to the gate (node G) of the first transistor (MN2) through the fourth and fifth transistors (MN1 and MN3).
According to a display device in which a shift register in a gate line drive circuit is formed of amorphous silicon TFT (a-Si TFT), a large area can be provided and productivity is high, so that it is widely employed in a screen of a notebook PC or a large screen display device. However, according to the a-Si TFT, when a gate electrode is positively biased continuously, the threshold voltage is likely to be positively shifted and its driving ability (ability to flow a current) is lowered. In addition, it is known that the threshold voltage is shifted not only in the a-Si TFT but also in an organic TFT.
When the unit shift register in FIG. 7 of National Publication of Translation No. 11-502355 performs the forward shift, since the first voltage signal (Vb) is at H level and the second voltage signal (Vn) is at L level, the fourth transistor (MN 1) charges the node G, and the fifth transistor (MN3) discharges the node G. Meanwhile, when the backward shift is performed, since the first voltage signal (Vb) is at L level and the second voltage signal (Vn) is at H level, the fifth transistor (MN 3) charges the node G, and the fourth transistor (MN1) discharges the node G.
When the unit shift register is used in the gate line drive circuit of the image display device, its each-stage output signal becomes H level once with respect to each frame period of the video signal. For example, at the time of forward shift (forward scanning), the fifth transistor (MN3) in each stage is turned on once per one frame period during a period (active period) while the output signal of the subsequent-stage (n+1) is at H level, and discharge the node G. Although the active period at each stage is very short (a few thousandths of one frame), the voltage between gate and the source of the subsequent-stage fifth transistor (MN3) is positively biased during that period and the threshold voltage is slightly shifted to the positive side. When this is repeated for a long period of time, the slight shift of the threshold voltage can be accumulated to several volts finally.
Since the fourth transistor (MN1) at the time of forward shift charges the node G in a source follower operation, the voltage between the gate and source thereof is as low as the threshold voltage, so that the threshold voltage is not shifted.
While the fifth transistor (MN3) performs the discharging operation, the shift of the threshold voltage does not affect that discharging operation. However, when the operation of the gate line drive circuit is switched to the backward shift (backward scanning) after that, the following problem arises.
The fifth transistor (MN3) at the time of backward shift charges the node G in the source follower operation similar to the fourth transistor (MN1) at the time of forward shift. Thus, the potential of the charged node G is a value dropped from the gate voltage of the fifth transistor (MN3) by its threshold voltage. Therefore, if the threshold voltage of the fifth transistor (MN3) has been shifted at the time of previous forward shift, the potential of the node G after charged is lowered by the shifted amount. As a result, the operation margin of the unit shift register is lowered and an error operation is likely to be generated.